The purification process in the instant invention applies to aromatic polycarboxylic acids having one or more condensed aromatic rings with two or more carboxylic acid groups at any position of the aromatic ring or rings. Typical examples of one-ring polycarboxylic acids are terephthalic acid, isophthalic acid and trimellitic acid; two ring aromatic polycarboxylic acids, 2,6-naphthlene dicarboxylic acid, 2,7-naphthalene dicarboxylic acids; three ring aromatic polycarboxylic acids, 2,3,6-anthracene tricarboxylic acid, etc. The background of terephthalic acid purification process is discussed first because of its largest commercial production quantity and it is the most difficult to purify due to its low solubility in most solvents, high boiling point, and similarities in physical and chemical properties with impurities present. Those skilled in the art would recognize that most principles discussed below for terephthalic acid are also applicable to other aromatic polycarboxylic acids.
Terephthalic acid has been well established as a starting material for manufacture of polyester fibers, films, and molding resin. However, the presence of its major impurities: p-toluic acid, benzoic acid, or 4-carboxybenzaldehyde (4-CBA), even in minute amounts, will adversely affect the quality of the polyester product from polymerization of terephthalic acid with ethylene glycol (MEG) into poly-ethylene terephthalate (PET). The impurities, such as monoffinctional p-toluic acid and benzoic acid, act as polymerization terminators that slow down the polymerization rate and decrease the average molecular weight of the polymer. Some other impurities, such as 4-CBA, cause undesirable coloring of the polymer as a consequence of their thermal instability during polymerization.
The purity specification for technical-grade terephthalic acid is 98.5+ wt %. However this purity is not high enough to be used as raw material for polyester production. Before polymer-grade, or pure terephthalic acid (PTA), can be commercially produced in 1965, technical-grade terephthalic acid was mainly used to produce polymer-grade dimethyl terephthalate (DMT) for its easier purification from crystallization and distillation. Either DMT or PTA is reacted with MEG to form bis(2-hydroxyethyl) terephthalate (BHET) which is condensation-polymerized to PET.
Polymer-grade terephthalic acid, i.e. PTA, must conform to many specifications to be suitable for the production of polyester fibers, films, and molding resin. Although no industry standards have been established officially, most polymer-grade terephthalic acids have maximum 25 ppm of residual 4-CBA and 150 ppm of p-toluic acid. Residual benzoic acid is generally low and not specified. However, significant amount of benzoic acid may still be present in some polymer grade terephthalic acids.
Currently, almost all technical-grade terephthalic acid is produced by catalytic liquid-phase air oxidation of para-xylene. This and other similar reactions producing crude aromatic polycarboxylic acids are referred to from time to time as oxidation reaction or oxidation reactions in this patent. Mid-Century process is the most widely adopted process which uses acetic acid as a solvent to assist slurry mixing and circulation; heavy metals, e.g., cobalt and manganese, as catalysts; and a bromine-containing compound as promoter. Reaction conditions are generally in the range of 175-230.degree. C. and 1500-3000 kPa. A variant of the process uses acetaldehyde as oxidation promoter that runs at 120-175.degree. C. and 700-1400 kPa. Some currently obsolete commercial processes are:
HNO.sub.3 oxidation of para-xylene(PX). The process was used by du Pont in the U.S. and ICI in the United Kingdom. PA1 Henkel I and II processes that rearrange benzoic acid or phthalic anhydride into terephthalic acid using naphthalene or toluene as starting material. These processes were used by Teijin, Kawasaki, and Mitsubishi in Japan. PA1 1. N,N-dimethylacetamide, or N,N-diemthylformamide, or their mixtures with water or methanol (U.S. Pat. No. 2,811,548); PA1 2. Pyridine with isopropylamine and others, recrystallizing in ethylene glycol, and acidifying in acid water (U.S. Pat. No. 2,829,160); PA1 3. N-formyl morpholine, or N-formnyl piperidine (U.S. Pat. No. 2,849,483); PA1 4. Ammonia with methanol and acetone (U.S. Pat. No. 2,862,963). PA1 1. Cooling to precipitate an amine salt of NDA, and the amine is then recovered from the amine salt by heating, thereby to obtain NDA having high purity. PA1 2. Cooling to precipitate an amine salt of NDA, and the amine is then treated with an acid, thereby to obtain NDA having high purity. PA1 3. Adding an acid to the solution, thereby to obtain NDA having high purity. PA1 Crude aromatic polycarboxylic acid may contain insoluble impurities that can be separated by using any suitable method, such as filtration, centrifugation, sedimentation, magnetic separation, evaporation, and others. PA1 The solution may be treated with an activated carbon or other suitable adsorbents by: PA1 (a) Changing the composition of the solvent used to dissolve the crude aromatic polycarboxylic acid by removing lower-boiling components in the mixed solvent by flashing under reduced process pressure, by evaporating at constant or variant temperatures, by distillation, or by adding more co-solvent, to precipitate the solid formed between aromatic polycarboxylic acid and the major solvent. In the case of evaporation or flashing, the solid is precipitated mainly due to the change of solvent composition and the reduction of solvent quantity. PA1 (b) Separating the precipitated solid followed by treating the separated solid with an acid solvent to obtain a purified aromatic polycarboxylic acid. PA1 (a) The same process as 1 (a) is used to precipitate a solid. PA1 (b) Separating the precipitated solid followed by heating the separated solid to a higher temperature, with or without purging with a non-oxidizing or inert gas such as N.sub.2, CO.sub.2, CO, He, Ar, H.sub.2, and others, to decompose the solid to obtain a purified aromatic polycarboxylic acid. PA1 (a) A crude aromatic polycarboxylic acid is dissolved in a solvent at elevated temperature. The solution is cooled to a lower temperature to precipitate the solid formed between aromatic polycarboxylic acid and the major solvent. The solid is precipitated mainly due to the change of temperature in solution. PA1 (b) The solid is separated and then treated with an acid solvent to obtain a purified aromatic polycarboxylic acid. PA1 (a) The same process as 3 (a) is used to precipitate a solid. PA1 (b) The solid obtained is separated and then heated to a higher temperature, with or without purging with a non-oxidizing or inert gas, such as N.sub.2, CO.sub.2, CO, He, Ar, H.sub.2, and others, to decompose the solid to obtain a purified aromatic polycarboxylic acid. PA1 To [a] substitute the solvent used in the crude aromatic polycarboxylic acid producing oxidation reactor with less corrosive materials, such as benzoic acid, methyl benzoate, ethyl benzoate, and phenyl benzoate, [b] use less amount of oxidation promoters, [c] use different kind of promoters, [d] reduce the severity of operating condition by reducing reaction temperature to 100 to 175.degree. C., [e] use a combination of the aforementioned [a] through [d] in order to use cheaper construction material, such as 316 SS, in the oxidation reactor or other parts of the process for lower capital investment. PA1 To run the oxidation reaction producing the crude aromatic polycarboxylic acid at a lower severity to reduce combustion loss of feedstock and acid solvent and to recycle the un-reacted feedstock back to oxidation reaction vessel to increase the overall yield and production efficiency. PA1 To process crude aromatic polycarboxylic acids from processes that are less efficient in producing high-purity product but cheaper in initial capital investment or operation cost. Examples are Henkel processes, HNO.sub.3 oxidation of PX, the terephthalic acid or NDA from DMT or NDC process respectively, etc. Impurity contents obtained from such esterification processes may be as high as 30 wt %.
Compared to DMT, advantages of polymer-grade terephthalic acid as a feedstock for PET are its lower cost, no methanol as by-product, lower investment and energy costs, higher unit productivity, and purer polymer because less catalyst is needed for the polymerization process. These factors, together with competitive marketing pressures, have induced a number of companies into developing processes that produce polymer-grade terephthalic acid since 1965. The success in the removal of impurities from technical-grade terephthalic acid has made polymer-grade terephthalic acid as a major and often the preferred feedstock for PET.
To produce polymer-grade terephthalic acid, separate purification processes have been developed to remove 4-CBA, p-toluic acid, and benzoic acid. The PTA process is separated into two sections: oxidation reaction section and purification section. The oxidation reaction section is for the production of technical grade or crude terephthalic acid (CTA) which is then introduced to purification section for removal of impurities. As discussed above, CTA production is generally produced by a liquid phase oxidation process. Terephthalic acid is also present as a major constituent in intermediate streams of the DMT production processes.
With slight variations, the prior art teaches that the purification section removes 4-CBA from terephthalic acid by chemically converting 4-CBA into p-toluic acid through hydrogenation reaction using charcoal supported noble metals, such as platinum, palladium, and so on, as the catalysts. P-toluic acid (converted from 4-CBA or existing in terephthalic acid as contaminant) is generally removed by recrystallizing terephthalic acid from water at elevated temperatures and pressures. An alternative is continuing further oxidation of 4-CBA to terephthalic acid.
However, this type of purification method, either by hydrogenation or oxidation, although efficient, can only handle relatively small amount of 4-CBA present in CTA initially. To meet the final polymer-grade PTA product specification, 4-CBA, the principal impurity present in CTA, is generally limited to less than 1.0 wt %, preferably less than 0.5 wt %, to avoid overloading the purification section of the process.
To achieve this purpose of reducing the amount of impurities introduced into the purification section, either the equipment has been modified to run at higher severity, or additional processing steps are added after the oxidation reaction step, such as a secondary oxidation step or reslurrying CTA in fresh acetic acid. Because higher severity increases the combustion rate of para-xylene, the CTA feedstock, and acetic acid, the preferred solvent, to CO and CO.sub.2, the overall yield of the desired product and production efficiency are both reduced. Using acetic acid as solvent and operating under severe condition require reactors and some other parts of the process to use expensive corrosion resistant material, such as titanium. This requirement increases initial capital investment significantly. Adding more processing steps likewise requires higher capital investments. Therefore, most prior arts teach the use of a relatively cumbersome purification procedure and high-cost equipment to remove as little as 0.5 wt % of impurities from terephthalic acid.
In searching for alternative methods to produce polymer-grade terephthalic acid, earlier patents disclosed that terephthalic acid could be purified by crystallization in organic solvents. A partial list of those solvents is given below
These disclosed organic solvents, however, have several disadvantages. They are unable to produce the required high purity terephthalic acid. They are either unstable themselves or tend to form additional products with terephthalic acid. It is also difficult to separate the residual solvent included in the crystals of the product.
On the other hand, the thermally more stable and chemically much less reactive solvents such as acetic acid, acetic anhydride (U.S. Pat. No. 3,574,727) and water, suffer from low solubility of terephthalic acid and lack of selectivity between terephthalic acid and 4-CBA. With this type of solvents, expensive hydrogenation process is required to convert 4-CBA into p-toluic acid most of which can be later removed by recrystallization in water (U.S. Pat. No. 3,584,039).
The manufacturing processes of isophthalic and phthalic acid that have the two carboxylic acids located at meta and ortho positions are similar to the manufacturing process of terephthalic acid. Liquid-phase oxidation production facilities often can be used interchangeably between terephthalic acid and isophthalic acid. Phthalic acid produced by this liquid process has significantly higher yields than those from vapor-phase oxidation processes with higher capital costs.
The manufacturing process of benzenetricarboxylic acid is also similar to the terephthalic acid process. Trimellitic acid is produced commercially in large volume in the U.S. mainly by liquid-phase air oxidation of pseudocumene. It is dehydrated to trimellitic anhydride, a preferred form commercially. Trimellitate esters have many superior properties than phthalic acid esters in certain applications. For example, trimellitate esters are used as plasticizers for poly-vinyl chloride, especially if permanency is required, e.g., in high temperature wire insulation. Other important uses of trimellitate esters are in alkyd resins, amide-imide polymers, and epoxy curing.
The manufacturing process of aromatic polycarboxylic acids with two condensed rings, such as naphthalene dicarboxylic acids (NDA), is also similar to terephthalic acid process. 2,6- or 2,7-NDA can be produced by the oxidation of 2,6- or 2,7-dialkyl naphthalene respectively with air or oxygen enriched air, in the presence of cobalt, manganese, and bromine. Relative to PTA, 2,6-NDA imparts greater structure stability to the resulting polymers at the same molecular level. Since the crude NDA also contains impurities, such as trimellitic acid, bromo-2,6-NDA, 2-naphthoic acid, 2-formyl-6-naphthoic acid, a similar purification process is required.
To improve NDA purity, a number of Japanese patents described the methods of dissolving the crude NDA in an aqueous solution of alkali, then subjecting the solution to such treatment as oxidation, hydrogenation, decoloring by adsorption, and so on, and followed by acidifying the resulting solution, thereby obtaining the purified NDA (JP-A48-68554, JP-B-52-20993, JP-A-50-105639, and JP-A-50-16024). However, the above methods suffer several drawbacks, that large amounts of acid and alkali have to be used, that an inorganic salt is produced, and that waste water is discharged in large quantities.
Organic solvents were also disclosed for purifying crude NDA as described by JPA-62-230747. An organic solvent selected from N,N-dimethylformamide (DMF), N,N-dimethylacetamide, and dimethyl sulfoxide (DMSO), treating the solution with active carbon, and then recrystallizing the purified NDA. However, the solubility of NDA in DMF or DMSO is low, so large quantity of solvent has to be used. Furthermore, it was found, in a higher NDA recovery mode, almost no improvement in color was achieved in the purified NDA product. Toxicity of the solvents is also a major concern. In addition, they are difficult to recover due to their high boiling points (U.S. Pat. No. 5,344,969).
An aqueous solution of alkylamines such as dimethylamine, was used to dissolve crude NDA and the purified NDA was precipitated by removing dimethylamine from the solution by distillation (JP-A-50-142542). However, a large portion of water in the aqueous solution is lost along with the amine because the amine evaporates as an azeotropic mixture with water. NDA recovery by the method is low because complete removal of the amine from the aqueous solution is extremely difficult.
Another alternative was to use alkylamines and alcohols to dissolve crude NDA. The purity and color of NDA were improved by precipitating NDA solids from the solution by one of the following precipitation method (U.S. Pat. No. 5,344,969):
The prior arts of purification by crystallization used either a pure organic solvent or a mixture of solvents at constant composition to dissolve crude aromatic polycarboxylic acids at high temperature. The solution was then cooled to precipitate solids and leave impurities in the solution to purify the acids. These processes mainly took advantage of differences in solubility at different temperature to dissolve the crude acids and precipitate the solids. Since the solubility of the crude acids in these solvents is generally insignificant around room temperature (25.degree. C.), the processes were focused to find a solvent or a solvent mixture having high solubility for the crude acids at high temperature. The solvent of course has to meet additional requirements, such as non-reactive with the crude acids, easy to be recovered, and extremely low amount of residual solvent to be remained in the purified product, etc.